I am sorry that your dog is having problems. This sounds very much like a spinal cord issue. There could be a herniated disc, a growth/mass in the vertebra or spinal cord itself, or possibly an injury to the vertebra or spinal cord. Of course there are other potential causes, but these would be on the top of my list.
My recommendation would be to go to you veterinarian for a good exam and some xrays. They may even recommend some advanced testing like MRI or CT if that is a route you would wish to persue. Sometimes these problems can be helped with rest and appropriate treatment. If the problem has progressed to no deep pain response (a test your veterinarian will perform in their exam) and has been that way for greater than 24hrs, there will be little chance of any function being regained.
If your girl is pain free, eating, drinking, able to go to the bathroom, and otherwise healthy except for the paralysis, and there is no treatment to improve her condition, there are carts that support the rear end and allow them to get around. If this is a bigger issue (cancer, painful fracture, etc.), you will have to discuss prognosis and the best course of action with you veterinarian.
I hope this information has been helpful, and I wish you the best of luck,
The other thing I forgot to mention was she has gain alot of weight the last couple of months and don't think we changed her feeding habit much
If she is truly paralyzed in both her hind limbs, I doubt it would be the only cause, but thyroid disease could cause increased weight gain with no increase in food or decrease in exercise. Dogs typically get hypothyroidism which slows metabolism, so they don't burn what they eat as quickly. They can also have some fur loss, ear and skin infections, fur doesn't grow back as quickly if shaved. They can also have muscle weakness and neurologic issues with this disease. Usually not severe enough to cause paralysis, however. I would still suspect a spinal cord lesion of some sort.
If there is hypothyroidism involved, supplementing with thyroid hormone pills will help with that and the weight gain. If it is a ruptured vertebral disc(slipped disc), sometimes rest, anti-inflammatories +/- muscle relaxants can improve function. If there is disc material still within the spinal cord, sometimes surgery is indicated. If there is a mass within the spinal cord or vertebral column, sometimes chemotherapy or radiation can be helpful, but the prognosis is poor. If pain can be controlled (If there is any), she is eating and drinking and happy, there is no cancer, and function can't be improved or returned, there are carts that strap to a dogs middle, support the back end and allow the dog to move around. A dog paralyzed in the rear end, especially a big dog, will require a lot of nursing care and responsibility. They can get sores and urine scald, and it can be difficult to get them about. Sometimes, it is necessary to consider quality of life issues when deciding what course of action to take.
Hope that helps.....